Compressed TCP/IP-Sessions using SSH-like tools
Sebastian Schreiber
2.2.2000
1. Introduction
In the past, we used to compress files in order to save disk space.
Today, disk space is cheap - but bandwidth is limited. By compressing
data streams, you achieve two goals:
1) You save bandwidth/transfered volume (that is important if you have
to pay for traffic or if your network is loaded.).
2) Speeding up low-bandwidth connections (Modem, GSM, ISDN).
This HowTo explains how to save both bandwith and connection time by
using tools like SSH1, SSH2, OpenSSH or LSH.
2. Compressing HTTP/FTP,...
My office is connected with a 64KBit ISDN line to the internet, so the
maximum transfer rate is about 7K/s. You can speed up the connection
by compressing it: when I download files, Netscape shows up a transfer
rate of up to 40K/s (Logfiles are compressable by factor 15). SSH is a
tool that is mainly designed to build up secure connections over
unsecured networks. Further more, SSH is able to compress connections
and to do port forwarding (like rinetd or redir). So it is the
appropriate tool to compress any simple TCP/IP connection. "Simple"
means, that only one TCP-connection is opened. An FTP-connections or
the connection between M$-Outlook and MS-Exchange are not simple as
several connections are established. SSH uses the LempleZiv (LZ77)
compression algorithm - so you will achieve the same high compression
rate as winzip/pkzip. In order to compress all HTTP-connections from
my intranet to the internet, I just have to execute one command on my
dial-in machine:
ssh -l -C -L8080::80 -f sleep
10000
= host that is located at my ISP. SSH-access is required.
= my login-ID on =the web proxy of my ISP
My browser is configured to use localhost:8080 as proxy. My laptop
connects to the same socket. The connection is compressed and
forwarded to the real proxy by SSH. The infrastructure looks like:
64KBit ISDN
My PC--------------------------------A PC (Unix/Linux/Win-NT) at my ISP
SSH-Client compressed SSH-Server, Port 22
Port 8080 |
| |
| |
| |
|10MBit Ethernet |100MBit
|not compressed |not compressed
| |
| |
My second PC ISP's WWW-proxy
with Netscape,... Port 80
(Laptop)
3. Compressing Email
3.1. Incoming Emails (POP3, IMAP4)
Most people fetch their email from the mailserver via POP3. POP3 is a
protocol with many disadvantages:
1. POP3 transfers password in clear text. (There are SSL-
implementations of POP/IMAP and a challenge/response
authentication, defined in RFC-2095/2195).
2. POP3 causes much protocol overhead: first the client requests a
message than the server sends the message. After that the client
requests the transferred article to be deleted. The server confirms
the deletion. After that the server is ready for the next
transaction. So 4 transactions are needed for each email.
3. POP3 transfers the mails without compression although email is
highly compressible (factor=3.5).
You could compress POP3 by forwarding localhost:110 through a
compressed connection to your ISP's POP3-socket. After that you have
to tell your mail client to connect to localhost:110 in order to
download mail. That secures and speeds up the connection -- but the
download time still suffers from the POP3-inherent protocol overhead.
It makes sense to substitute POP3 by a more efficient protocol. The
idea is to download the entire mailbox at once without generating
protocol overhead. Furthermore it makes sense to compress the
connections. The appropriate tool which offers both features is SCP.
You can download your mail-file like this:
scp -C -l loginId:/var/spool/mail/loginid /tmp/newmail
But there is a problem: what happens if a new email arrives at the
server during the download of your mailbox? The new mail would be
lost. Therefore it makes more sense to use the following commands:
ssh -l loginid mailserver -f mv /var/spool/mail/loginid
/tmp/loginid_fetchme
scp -C -l loginid:/tmp/my_new_mail /tmp/loginid_fetchme
A move (mv) is a elementary operation, so you won't get into truble if
you receive new mail during the execution of the comands. But if the
mail server directories /tmp/ and /var/spool/mail are not on the same
disc you might get problems. A solution is to create a lockfile on the
server before you execute the mv: touch /var/spool/mail/loginid.lock.
You should remove it, after that. A better solution is to move the
file loginid in the same directory:
ssh -l loginid mailserver -f mv /var/spool/mail/loginid
/var/spool/mail/loginid_fetchme
After that you can use formail instead of procmail in order to filter
/tmp/newmail into the right folder(s): formail -s procmail <
/tmp/newmail
3.2. Outgoing Email (SMTP)
You send email over compresses and encrypted SSH-connections, in order
to:
ˇ Save network traffic
ˇ Secure the connection (This does not make sense, if the mail is
transported over untrusted networks, later.)
ˇ Authenticate the sender. Many mail servers deny mail relaying in
order to prevent abuse. If you send an email over an SSH-
connection, the remote mail server (i.e. sendmail or MS-exchange)
thinks to be connected, locally.
If you have SSH-access on the mail server, you need the following
command:
ssh -C -l loginid mailserver -L2525:mailserver:25
If you don't have SSH-access on the mail server but to a server that
is allowed to use your mail server as relay, the command is:
ssh -C -l loginid other_server -L2525:mailserver:25
After that you can configure your mail client (or mail server: see
"smarthost") to send out mails to localhost port 2525.
4. Thoughts about performance.
Of course compression/encryption takes CPU time. It turned out that an
old Pentium-133 is able to encrypt and compress about 1GB/hour --
that's quite a lot. If you compile SSH with the option "--with-none"
you can tell SSH to use no encryption. That saves a little
performance. Here is a comprise between several download methods
(during the test, a noncompressed 6MB-file was transfered from a
133MHz-Pentium-1 to a 233MHz Pentium2 laptop over a 10MBit ethernet
without other load).
+-------------------+--------+----------+-----------+----------------------+
| | FTP |encrypted |compressed |compressed & encrypted|
+-------------------+--------+----------+-----------+----------------------+
+-------------------+--------+----------+-----------+----------------------+
| Elapsed Time | |7.6s | 26s | 9s | 23s |
+-------------------+--------+----------+-----------+----------------------+
| Throughput | 790K/s | 232K/s | 320K/s | 264K/s |
+-------------------+--------+----------+-----------+----------------------+
|Compression Factor | 1 | 1 | 3.8 | 3.8 |
+-------------------+--------+----------+-----------+----------------------+
5. Greetings
Thanks to Harald König , who used
rcp in order to download complete mailboxes. The latest version of
this howto is available on http://www.syss.de/howto.